Vehicle electrical systems in the form of multivoltage vehicle electrical systems are becoming increasingly important in motor vehicles. Vehicle electrical systems may include an electrical generator (generally an alternating current generator, although the use of direct current generators is also known) which provides the voltage(s) for the vehicle electrical system by converting mechanical kinetic energy into electrical energy.
Due to ever-increasing demand for electrical energy (due to need-based electrification of previously uncontrolled, mechanically or hydraulically operated consumers) and the intention to convert what may be a large quantity of kinetic energy into electrical energy during deceleration phases of a motor vehicle, an enhanced vehicle electrical system voltage (of 48 V in particular) is becoming established. With such a vehicle electrical system voltage, which is approximately four times higher than conventional, lower vehicle electrical system voltages (12 V), on the one hand currents which are four times lower, and on the other hand power which is four times higher, in comparison to conventional vehicle electrical system voltages may be achieved. An upper limit of the vehicle electrical system voltage is imposed by a maximum permissible voltage of 60 V in the vehicle electrical system, which represents the limit for touch voltages which are hazardous to humans, above which (in the event of exceedance) additional complex, costly measures become necessary (for example, warning signs, orange cables, double insulation, insulation monitoring, etc.).
Since numerous consumers are present in motor vehicles, which on the one hand are not easily and cost-effectively convertible to a higher vehicle electrical system voltage, and on the other hand cannot derive a technical/monetary benefit from the higher vehicle electrical system voltage (for example, the radio, dashboard, power windows, etc.), the vehicle electrical system may be configured as a multivoltage vehicle electrical system, in particular as a dual voltage vehicle electrical system including a low-voltage vehicle electrical system having a first, lower vehicle electrical system voltage (in particular 12 V or 24 V), and a high-voltage vehicle electrical system having a second, higher vehicle electrical system voltage (48 V).
In these types of multivoltage vehicle electrical systems, there is the risk of undesirable crosstalk from the high-voltage vehicle electrical system into the low-voltage vehicle electrical system, which could cause destruction of all low-voltage components (which generally have a maximum electric strength of 34 V/500 ms).
One option for preventing crosstalk is to provide galvanically separated ground lines of the 48-V vehicle electrical system (CL41) and of the 12-V vehicle electrical system (CL31) in order to reliably exclude possible coupling or further destruction in the event of ground interruptions. A galvanic separation may take place capacitively (specialized ISO components, for example), optically (optical couplers, for example), or inductively (HF transformers, for example). In the automotive field, all these methods share the common feature that they are complex and costly (due, among other factors, to high demands on the service life and the temperature range, as well as manufacture in small quantities).
The space requirements and the increase in complexity are likewise not insignificant. On the other hand, the above-mentioned galvanic separators generally provide electric strengths well above the required 60 V (generally up to 1,000 V or 1,200 V), since they are used nowadays primarily for 230 VAC (power grid) or 400 V to 600 VAC (hybrid). They are thus greatly oversized and correspondingly expensive.
In addition to the disadvantages due to the effort for galvanic separation, as the result of implementing the requirement for separate ground lines for a generator application (high-voltage generator, for example the boost recuperation system (BRS)), unlike the related art for automotive generators, it would no longer be allowable or possible to use the engine ground (via the fastening screws) as ground, which instead would have to be insulated, with corresponding effort, from the engine block. This is disadvantageous in particular with regard to costs and robustness. In addition, a dedicated ground connecting bolt would have to be provided on the generator, which in addition to the extra costs and installation space problems, in principle involves the risk of polarity reversals. In addition, this dedicated ground connecting bolt would have to be separately connected to the chassis ground, which would require an appropriately current-resistant (and therefore thick and costly) ground strap. The length of the ground strap to be expected would in turn make EMC measures, for example common mode filters, necessary. At the high power levels (high currents) which make the provision of a 48 V vehicle electrical system necessary or meaningful in the first place (in the BRS, for example 250 A to 300 A), relevant voltage drops result at the common mode filter (power losses) which reduce the efficiency gain (loss of performance).
It is therefore desirable to be able to obtain the engine block as a shared ground connection, the electric machine for supplying the high-voltage vehicle electrical system being fastened to ground on the engine block, which likewise defines the ground potential for the low-voltage vehicle electrical system. In the case of a so-called ground interruption (disconnection of the ground connection between the engine block and chassis, which defines a reference ground potential), without further measures an overvoltage results in the low-voltage vehicle electrical system.
It is therefore desirable to provide an option for preventing objectionable crosstalk from a high-voltage vehicle electrical system into a low-voltage vehicle electrical system of a multivoltage vehicle electrical system.